Or the way a railgun firing a piece of plastic can demolish a brick.

Uh... now you're bringing size and dissimilarity of objects into the equation. Ignoring that, though, have you ever looked at the piece of plastic after it's shattered the brick? Brick -- rigid, no method of dissipating force. Plastic -- elastic, and could deform and revert to shape. Ever dug bullets out of something, and notice how they're not all nice and pointy shaped anymore (d33zy can back me up on this one, I'm sure. )? Then there's pressure to think of, too -- objects that collide with a large surface area of impact will have less damage over a greater area than two objects that collide with a tiny surface area of impact. The smaller surface area of impact is going to create much higher pressure on one of those objects. Here's an experiment you can do to demonstrate the idea I'm talking about, since I don't think I'm doing a good job explaining it... get a fork, and hold it in one hand. Swing the hand holding the fork at your other arm, striking your arm with the flat tines of the fork. Take note that it doesn't hurt that much. Now, swinging the hand with the fork at the same rate you did last time, stab your arm with the pointy ends of the tines. Take note of how much it hurt that time, while the only thing that changed was the surface area of impact. If you want to use similar materials, think of poking someone with your finger as opposed to pushing with the flat of your palm. Same hand, same arm, same momentum, different area of impact.

The answer to physics problem #2 can only hold true in a "perfect physics world". Outside that world, you have to worry about all those silly things like minute variations in angle of impact, size of impact area on each vehicle, and the like are all going to govern the amount of damage that each vehicle sustains in the collision.

The reason I chose to use the example of the jello and granite was to show that it doesn't matter which of the two objects is moving, the jello is going to sustain the damage (although, now that I think about it some, jello may be too elastic to suffer damage in the collision, and just wobble around a bit).

I doubt the brick would suffer much damage if it were fired at the same speed at a small piece of plastic. It's just a matter of inertia.

If the plastic bit was moving free, of course not. If you were to fix the plastic bit in position, it would. No matter, it's irrelevant to the question at hand -- which of two identical bodies would suffer the most damage. With identical objects, they will have identical inertia, which is proportional to an object's mass, and is unrelated to the current velocity of the object. A stationary object has the same inertia as it would if it were moving at 30mph.

FWIW, I gave up on physics after first year university (I was passing, but not enjoying it), so if I'm bungling my explanations, speak out...